(1) Field of Invention
THIS INVENTION relates to the dynamic monitoring and control of jigs.
(2) Prior Art
The University of Queensland has patented and developed a control system for jigs that is centred on the concept that the jig can be controlled in response to measurement of the time variation of a signal during a cycle for the jig pulsation. The idea of signal averaging over a number of jig cycles is also invoked to provide more accurate signals and the Patent refers particularly to the measurement of density in the jig bed as a function of time. The Abstract of Australian Patent No. 596858 (AU-B-76489187) states:
"The density of the material in the jig bed is measured in consecutive short segments over the jig cycle, the time period of each segment not being greater than one-tenth the cycle time of the jig, to determine the density signature or profile of the jig. By controlling the operating parameters (e.g. inlet and outlet valve opening and closing, underbed water flow rate, discharge gate position and jig working air (pressure) of the jig, the density signature or profile is maintained within a control envelope for efficient stratification of the mineral."
It is relevant to outline some basic aspects of the physics of operation of jigs, including centrifugally-aided jigs also known as Kelsey jigs. These concepts are also applicable within minor modification to moving screen jigs.
The separation in a jig occurs as a result of the passage of a pulsating flow of water through a bed of particles that are supported on a screen or punched plate. As the water flows through the screen and into the bed of particles (pulsion phase of flow--consider such a flow direction to be positive or `upward`), the fluid drag on the particles supports more and more of the weight of the bed of particles until at some critical velocity, the bed of particles lifts. Further increase in fluid velocity causes fluidisation and dilation (decrease in the volume fraction of solids) of the bed. As the pulsion phase of the flow finishes and the fluid velocity reverses and increases in velocity downwards, the dilation of the bed rapidly decreases and the bed is forced back against the bed plate. Further increase in the downward velocity causes a significant pressure drop to develop across the bed.
Superimposed on this pulsating flow is a relatively small constant upflow of water through the bed. The purpose of this flow is to supply a current of water above the bed plate that will assist in the transport of the particles in a direction perpendicular to the pulsing flow. In a continuous jig, the particles of coal or mineral are transported from the feed end of the jig to the discharge end and the bed becomes progressively more stratified or better separated or better sorted as the material moves from the feed end to the discharge end.
The extent of separation is gauged by the extent to which particles of different size and true particle density become sorted in the bed. For example, for a particle bed composed of only one size of particle, but having a range of particle densities, an arrangement of particles in the bed such that particle density decreases monotonically from the bed plate towards the top of the bed would be considered to be perfectly sorted or separated.
The manner in which particles of differing density move from one layer to another in the bed as separation progresses depends on the extent to which the particle bed dilates and the relative vertical motion of the particles in response to the fluid flow through the particle bed. When a broad distribution of particle sizes exists in the bed, particle motion can also occur in the bed by a trickling mechanism wherein small particles move through the interstices of the packed bed.
One must also distinguish between two general methods of jigging separation, namely `through the screen` separation or `over the screen separation`. In the first case, the bed becomes stratified or sorted with high density particles moving towards the bed plate and low density particles moving towards the top of the bed. If the apertures in the bed plate are sufficiently large, particles of high density material may pass through the plate into the space (known generally as the hutch) on the other side of the bed plate for collection. Generally, when high density particles are concentrated in this way, a quantity of relatively large relatively high density particles are added to the bed with the intention that these particles shall remain in the bed next to the bed plate and form a `ragging` bed through the interstices of which the smaller particles to be concentrated may move. In the second case of over the screen separation, no ragging is added and the entire mass of feed material is allowed to stratify under the influence of the pulsations. A means of splitting the bottom layers of the bed from the upper layers of the bed is provided at the discharge end of the jig so that high density and low density products are recovered from the separator.
Finally, one must note that through the screen and over the screen jigging mechanisms of separation can function simultaneously in a jig.